1. Field of the Invention
The present invention relates to a capacitive sensor that detects deformation and the like on the basis of electrostatic capacity variation between electrodes.
2. Description of the Related Art
With a capacitive sensor, compressive deformation can be detected using variation in electrostatic capacity accompanying variation in the distance between a pair of electrodes. For example, Japanese Patent Application Publication No. JP-05-288619-A and Japanese Patent Application Publication No. JP-2004-117042-A disclose capacitive sensors including a pair of electrodes that face each other via a space. Further, Japanese Patent Application Publication No. JP-2005-315831-A discloses a capacitive sensor in which electrodes, each of which is constituted by a conductive fabric, are arranged on both surfaces of a sheet dielectric body.
A metallic material is used for the electrodes of the capacitive sensor described in Japanese Patent Application Publication No. JP-05-288619-A. A metallic electrode is not stretchable. Therefore, when the electrode bends, for example, the electrode is likely to break due to plastic deformation. Further, when a capacitive sensor is configured such that a dielectric body, which is elastically deformable, is interposed between metallic electrodes, the dielectric body is bendable, but the electrodes cannot follow the deformation of the dielectric body. Therefore, the electrodes peel away from the dielectric body and cannot be used repeatedly. Hence, a capacitive sensor having metallic electrodes is not suitable for detecting bending deformation. In addition, attachment thereof to a curved surface is difficult.
Japanese Patent Application Publication No. JP-2004-117042-A states that the electrodes may be formed from a material other than metal. More specifically, according to the capacitive sensor described in Japanese Patent Application Publication No. JP-2004-117042-A, one of the electrodes is formed from conductive rubber and the other electrode is formed from conductive ink using a screen printing method.
However, in the capacitive sensors of both Japanese Patent Application Publication No. JP-05-288619-A and Japanese Patent Application Publication No. JP-2004-117042-A, an air layer is provided between the pair of electrodes. The electrostatic capacity (capacitance) of a capacitive sensor in which a dielectric layer is interposed between a pair of electrodes can typically be determined using the following Equation (1).C=∈0∈rS/d  (1)
[C: capacitance, ∈0: dielectric constant in a vacuum, ∈r: specific dielectric constant of the dielectric layer, S: electrode area, d: distance between electrodes]
As apparent from Equation (1), the capacitance (C) increases as the electrode area (S) increases with respect to a strain inputted from outside. Further, when the thickness of the dielectric layer, or in other words the distance between electrodes (d), decreases, the capacitance (C) increases. Here, as the specific dielectric constant (∈r) of the dielectric layer becomes large, the capacitance (C) output from the capacitive sensor increases. In other words, variation in the capacitance (C) between before and after a load is applied from the measurement subject to the capacitive sensor becomes larger.
However, in the capacitive sensors of Japanese Patent Application Publication No. JP-05-288619-A and Japanese Patent Application Publication No. JP-2004-117042-A, the specific dielectric constant of the air layer is small. Therefore, the detected electrostatic capacity is small. In other words, the detection sensitivity is low. Moreover, a high degree of dimensional precision of the distance between electrodes is required, and therefore manufacturing cost is high.
Meanwhile, in the capacitive sensor described in Japanese Patent Application Publication No. JP-2005-315831-A, a fabric that is electrically conductive (a conductive fabric) is used for the electrodes. The conductive fabric is stretchable. However, the stretching direction is limited by the weave. Furthermore, the conductive fabric takes a mesh form, and therefore the electrode area decreases in accordance with the gaps in the mesh, leading to a reduction in the electrostatic capacity.
Further, Japanese Patent Application Publication No. JP-05-288619-A discloses a sensor sheet over which a large number of sensor cells (capacitive sensors) are dispersed. When a load is applied to the sensor cells from the measurement subject, the thickness of the air layer varies. When the thickness of the air layer varies, the electrostatic capacity between the pair of electrodes varies. On the basis of this electrostatic capacity variation, the sensor sheet detects irregular forms and the like on the measurement subject as a surface pressure distribution. However, in the sensor sheet disclosed in Japanese Patent Application Publication No. JP-05-288619-A, one sensor cell is arranged in each of the portions in which the surface pressure is to be detected. Therefore, the number of sensor cells, and accordingly the number of electrodes, is large. Moreover, since the number of electrodes is large, a large number of conductors are required to detect the electrostatic capacity.